Electrodeless discharge lamp and power coupler therefor

ABSTRACT

An electrodeless discharge lamp, consisting of a transparent envelope containing an ionizing gas and a minor amount of metallic element for producing spectral light, is started and operated by r-f power coupled to the lamp by a resonant coupler that consists of a helically coiled wire conductor mounted concentrically within a grounded hollow cylinder which is open at one end with a grounded base member in the other end.

United States Patent Gabriel Mar. 25, 1975 ELECTRODELESS DISCHARGE LAMPAND POWER COUPLER THEREFOR Fred C. Gabriel, Stamford, Conn.

The Perkins-Elmer Corporation, Norwalk, Conn.

Filed: Feb. 11, 1974 Appl. No.: 441,600

Related U.S. Application Data Continuation of Ser. No. 337,060, March 1,1973, abandoned.

Inventor:

Assignee:

U.S. Cl. 315/267, 315/344 Int. Cl. H05b 41/24 Field of Search 315/248,267, 344, 348;

References Cited UNITED STATES PATENTS 4/1966 Booth et a1. 315/248 X .HMJ

warms Primary Examiner-James B. Mullins Attorney, Agent, or Firm-John K.Conant 12 Claims, 1 Drawing Figure ELECTRODELESS DISCHARGE LAMP ANDPOWER COUPLER THEREFOR This is a continuation, of application Ser. No.337,060, filed Mar. 1, 1973, and now abandoned.

BACKGROUND AND PRIOR ART The present invention is an electrodelessdischarge lamp with an improved coupler for coupling r-f (radiofrequency) electric power into the lamp to start and operate the lamp.Such lamps normally comprise a quartz bulb containing gas such as argon,and are operated by starting and maintaining a discharge in the ionizedgas.

In particular this invention is a power coupler for a electrodelessdischarge lamp in which a trace of a selected metallic element isincorporated to provide a bright spectral line source of the selectedmetal for use in chemical analysis, particularly atomic spectroscopy. Abright spectral line source provided in this manner might also be usefulfor optical pumping applications.

At present the basic light source for atomic spectroscopy are the hollowcathode lamps. For analyzing for most metallic elements hollow cathodelamps are sufficiently bright, stable, long-lived and inexpensive.However, hollow cathode lamps do not provide sufficiently bright orstable spectral lines for certain metallic elements, such as arsenic,cesium and rubidium, and for other elements such as selenium, tellurium,bismuth, antimony and tin, hollow cathode lamps are not bright enoughfor good spectral work. Electrodeless discharge lamps in general are analternative to hollow cathode lamps for the metallic elements with whichhollow cathode lamps are now used, but more importantly electrodelessdischarge lamps provide highly satisfactory sources of the spectrallines of the foregoing metallic elements which cannot be provided byhollow cathode lamps or which are barely suitable as provided by ahollow cathode lamp.

Moreover, when operated efficiently electrodeless discharge lamps arecapable of providing far brighter spectral line sources of metallicelements than hollow cathode lamps. While this is not of greatsignificance for present atomic spectroscopy techniques with whichbrightness above a level of sufficiency is not especially useful, thepractical availability of brighter spectral line sources might well makeimproved analytical techniques possible or enhance the usefulness ofknown techniques. For example, atomic fluorescence spectroscopy haslittle analytical usefulness at present, but since it has thecharacteristic that its detection limits improve almost linearlywithincreased source interisity, the practical availability of brightersources might transform it into a commercial useful analytical tool.

The use of electrodeless discharge lamps as spectral line sources havebeen known for a number of years. It is known to operate electrodelessdischarge lamps with high frequency power, e.g., 2,450 megacycles andalso with much lower frequency power of about 27 megacycles, i.e., radiofrequency. The lower radio frequency has the particular advantage ofproviding more stable operation of the lamp.

Typically r-f operated electrodeless discharge lamps known in the artare excited to maintain a discharge in the lamp by placing the lamp inthe final tank circuit inductor of a radio transmitter or poweroscillator and the r-f power is coupled magnetically to the lamp plasmaonce a discharge is started in the lamp. Start-up of the lamp requiresan initial voltage, higher than the voltage required to maintain adischarge after start-up, to ionize the gas and ignite a discharge inthe ionized gas. The start-up voltage supplied is typically a voltagetransient from an extended circuit or from the r-f supply itself;typical arrangements are described in an article titled Miniature DeviceFor Starting Electrodeless Discharge Tubes by J. K. Brady in the Reviewof Scientific Instruments, Volume 36, 1965, Page 710, and in an articleby M. M. Katsman, V. l. Konstantinov and S. M. Sutovskii, in the RussianJournal Zhurnol Priklodnoi Spectroskopii, Volume 6, Number 2, 1967,Pages 279 to 281.

Previously known and used power supplies for starting and maintaining adischarge in electrodeless discharge lamps are rather complex,relatively expensive and require special switching means for supplyingfirst the high voltage required for ionizing the gas and starting adischarge therein, and then for supplying a' reduced level of voltage tomaintain the discharge.

An object of the present invention is to provide a spectral sourceelectrodeless discharge lamp operating unit which includes an inductiveresonator coupler, for

a coupling r-f electric power into the lamp for starting and operatingit,,that provides efficient coupling, that is simple and economic toconstruct and operate, that provides the requisite voltage and currentratios for starting the lamp and then for maintaining it in operationwithout intermediate switching means or alternative power supplyconnections, and that has an automatic self starting capability if thelamp goes out during operation.

BRIEF SUMMARY OF THE lNVENTlON The invention is an electrodelessdischarge lamp, for use as a spectral light source, and an inductiveresonator coupler for coupling r-f electric power into the lamp forstarting and maintaining a discharge inthe lamp. The lamp is atransparent envelope, normally quartz, containing an ionizing gas,namely an inert gas such as argon, under low pressure and a minor amountof a metallic element whose spectral line is to be provided by the lamp.The coupler consists of a grounded hollow cylinder of electricallyconductive material open at one end, for light from the lamp to passout, and with a grounded base member in the other end. A helicallycoiled wire conductor, is mounted concentrically within the cylinder;the coil is a smaller diameter than the cylinder so as to provide anannular space between the coil and the cylinder wall. The length of wirein the coil is made nominally one quarter of the free-space wave lengthof the r-f electric power'to be applied. One end of the coil is groundedto the base member of the cylinder and the other end is unconnected,i.e., open circuited. The cylinder is longer than the coil with its openend extending beyond the open circuited end of the coil. The lamp issuitably supported on the base member of the cylinder and is mounted sothat the major portion of the lamp is within the coil at the groundedend of the coil. R-f electric power from an r-f generator, which may beof conventional design, is tapped into the coil at a point near thegrounded end of the coil. The tap point is selected so that when thelamp is operating the circuit formed by the coupler cylinder and coiland the lamp plasma, which is magnetically coupled by transformer actionto the coil turns between the tap and ground, are tuned to the frequencyof the r-f power and the impedance of the coupler and lamp match theimpedance of the r-f power supply at the tap.

This coupler provides two distinct modes of operation successively;first it applies the electrostatic field to the gas in the lamp forstarting the discharge and then it applies the electromagnetic fieldrequired for maintaining the discharge.

When the r-f power is first applied to the coupler, before the lampdischarge is started, the resistance and reactance reflected by the lampare absent so that the coupler has a much higher impedance than when thelamp is operating, and the coupler has a very high electrical Q. Theinput r-f voltage thus appears as a high voltage maximum at the opencircuited end of the coil.

This results in a high potential through the gas in they lamp betweenthe open circuited end of the coil and the grounded base member of thecylinder. This ionizes the gas to the point at which a discharge canstart. Sometimes the lamp will start by itself at this point, apparentlydue to stray energy reaching the lamp from surrounding light or the sun.In any event, very little extra energy is then required to initiate thedischarge; the extra light from a lighted match is sufficient. When thedischarge starts the coupler loses energy to the lamp the Q drops andthe impedance of the lamp and coupler circuit drops to the level atwhich there is an impedance match (resistive) with the r-f power supply.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is described in moredetail below with reference to an illustrative embodiment shown in theaccompaning drawing which is a side elevation, partly in section, partlybroken away and partly schematic, of a preferred form of the lamp andinductive resonant coupler in accordance with the invention.

DETAILED DESCRIPTION Referring to the drawing the invention comprisesgenerally an electrodeless discharge lamp and an inductive resonantcoupler 11 for coupling r-f electric power from an r-f generator 12 intothe lamp for operating the lamp.

The lamp [0, which is mounted within the structure of the coupler 11, isa small quartz bulb enclosing a volume of from I to 10 cubic centimeterscontaining an ionizing gas at low pressure, and a minor amount of ametallic element whose spectral line is to be produced by the lamp. Theionizing gas is one of the inert gases, argon being one that is commonlyused; the pressure within the lamp is from k to about 5 torr. Theparticular gas and pressure are not critical, the general considerationsbeing that the gas must be ionizable and support a discharge atreasonable power levels and the pressure must be such that, when thelamp is hot, the increased pressure will not be so large as toextinguish the discharge. The metallic element is present in microgramquantities; the particular amount is not critical, a minute trace willprovide the spectral line desired but enough more is normally providedto make up for some of the vaporized metallic element that is lost byparticles becoming embedded in the walls of the lamp. The spectral linesare the lines of the metals themselves and in the usual case, as witharsenic and selenium for example, the metallic element incorporated inthe lamp is the metal itself. With some metals, such as lead for whichthe metal itself would not provide the requisite vapor pressure toprovide the spectral line in the lamp output, a salt of the metal, e.g.,lead chloride is used.

The lamp 10 has a bulbous portion 10a and a stem portion 1017 which isutilized for mounting and positioning the lamp within the coupler 11 inthe manner subsequently to be described.

The inductive resonant coupler 11 is made up of a hollow cylinder 14 ofan electrically conductive material and a helical coil 15 wound on aceramic coil form 16 within the cylinder. One end of the cylinder 14,the right hand end, is open; its other end is closed by a base member 17which is also of electrically conductive material. The base member 17 isgrounded in the r-f gen erator as indicated at 18, and is fixed in theend of the cylinder so that the cylinder is electrically connected tothe base member and thus similarly connected to ground.

One end of the coil 15, the left hand end as shown, is grounded by beingconnected to the base member; its other end is unconnected and thus opencircuited. The length of the wire of coil 15 is made one quarter of thewave length of the rf electric power to be supplied by the r-f generator12. The diameter of the coil 15 is less than the inside diameter of thecylinder 14 so as to provide an annular air space 19 between them. Thecoil and cylinder are thus in a capacitor relationship. In practice, therelative diameters of the cylinder and coil are 2 inches and l inchrespectively so that the gap between is about one-half an inch. Thecylinder is preferably made slightly longer than the coil to provide asuitably long ground plane relative to the field of the last turn of thecoil at its open-circuited end to avoid unwanted fringe effects. In theembodiment shown the open end of the cylinder extends beyond the amountnecessary to avoid fringe effects and actually extends more than an inchbeyond the end of the coil as means to help in suppressing unwantedtransmissions.

The lamp 10 is mounted for its major portion, its bulbous portion 10a,to be within the portion of the coil at the grounded end. The lamp ismounted by means of its stem portion 10b which fits through a bushing 20that is fixed through the base end of the coil form 16 and the basemember 17 of the cylinder. The end of the bushing 20 within the coilform 16 is flanged as shown at 21 and this serves to hold the coil formin place on the base member 17. The lamp stem 10b extends out throughbushing 20 so that the position of the lamp bulbous portion 10a can beadjusted relative to the coil by moving the stem in or out relativelythrough the bushing. The position of the lamp relative to the coil isthus adjustable for finding the exact position at which the energysupplied by the coil, as subsequently described, is coupled into thelamp most efficiently when the lamp is operating. The position of thelamp is clamped by means of a cap 22 threaded onto the activated end ofthe bushing. The cap 22 has a hole through it for the lamp stem 10b toextend through and clamping means is provided by an O-ring capturedbetween the end of the bushing 20 and the inside of the end of the cap22. When the lamp position has been selected, the lamp is clamped byscrewing the cap 22 in to squeeze the O to frictionally clamp the lampstem 10b.

R-f electric power is applied to the coil 15 from the M" generator 12 bymeans of a connection from the generator 12 through a connector 25mounted through the cylinder base member 17 to a tap 26 into the cell.It is normally desired to have the lamp and coupler spaced some distancefrom the r-f generator as a means of eliminating heat problems and forflexibility of instrument design; in this case, as illustrated, the r-fgenerator 12 is shown connected to the connector 25 through a coaxialcable 27. The r-f generator is comparable to a conventional radiotransmitter, which would be a suitable r-f source; as shown it mayconsist of a crystal oscillator 28 coupled respectively through a lowpower r-f amplifier 30 and a high power r-f amplifier 31 to apply an r-foutput through the coaxial cable 27 and connector 25 to the coil tap 26.The r-f generator 12 is operated from a conventional source of electricpower, not shown. in practice the r-f generator is built to supply r-felectric power at 27.l2 MHz at levels up to 45 watts and having animpedance of 50 ohms. The coaxial cable 27 used would of course be a 50ohm cable.

The tap 26 into the coil 15 is located relative to the turns of the coilso that when the lamp is operating the coupler 11 is tuned to thefrequency supplied by the r-f generator and the impedance of the couplerand lamp circuit matches the impedance of the r-f generator at the tap26.

A trimming capacitor 32 may be provided between the wall of the cylinder14 and the coil 15, as shown, to assist in providing a fine tune foradjusting the resonant frequency of the coupler and lamp circuit, whenthe lamp is operating, to the resonant frequency of the r-f generator.This capacitor 32 may be eliminated by adjusting the resonant frequencyof the generator, or by building the coupler 11 with a precision whichmakes additional trimming unnecessary.

As noted above, the length of wire in the coil 15 is nominally onequarter of the free-space wavelength of the r-f power to be supplied bythe r-f generator 12. The coupler and lamp configuration of thisinvention thus provides a compact, quarter-wave, transmission lineresonator which can be proportioned to have extremely low losses.

The coupler 11 has, due to the general configuration shown, a highelectrical O. This is enhanced by making the coupler of low lossmaterials and reducing losses by all practical means. In practice the,cylinder 14 and base member are made of copper or brass and the interiorof the cylinder is plated with a polished silver coating indicated at33, and the wire of the coil 15 is solid silver or silver plated copperfor the purpose of maintaining high r-f conductivity in the presence ofhigh lamp temperatures and corrosive atmosphere. Also soldered taps areavoided, the taps preferably being welded.

When the power from the r-f generator 12 is first applied to the coupler11 and the lamp has not yet started a discharge, the resistance andreactance reflected by the lamp discharge when the lamp is operating areabsent so that the impedance. of the coupler circuit is higher than the50 ohm mentioned above, and is typically 400 to 500 ohms. Also becausethe lamp is not extracting power, the coupler has a very high electricalQ and the open circuited end of the coil 15 is the location of a currentnode and hence a voltage maximum. This has the effect of transformingthe input r-f voltage swing to a very high voltage (e.g., tens ofthousands) at the open circuited end of the coil. This high voltagecreates a potential through the gas in the bulbous portion 10a of thelamp to the grounded base member 17 of the cylinder which ionizes thegas. in many instances the electrostatic energy thus coupled into thelamp and a slight amount of additional energy applying to the lamp fromthe surrounding light, such as direct sunlight or light bulbs, is enoughto start the discharge in the ionized gas. In any event only a slightadditional amount of energy is necessary for starting the discharge andthis may be supplied by such simple means as a lighted match.

When the discharge starts, the Q of the coupler drops, the high voltagesubsides and the lamp is thereafter maintained in operation by power inthe turns of the coil 15 between the grounded end and the input tap 26magnetically coupling into the lamp. Ifthe discharge should extinguish,voltage builds up in the open circuited end of the coil again, anddischarge will then automatically reignite. When the lamp is operatingthe voltage drop back as just described and the impedance of the circuitof the coupler 11 and lamp 10 then matches the impedance of the r-fsupply at the tap 26 thereby providing highly efficient operation of thelamp. I

The lamp may be operated to produce a steady output, or the output maybe modulated for use with certain spectroscopy instruments, that areadapted to operate with a modulated light source, by modulating theoutput of the r-f generator between an upper output level and a lowoutput level, the low output level being one at which the lamp wouldstill be operating, but producing a light below a minimum levelrecognized by the spectroscopy instrument. In practice a 50 percentmodulation of the output of the r-f generator is suitable for adaptingthe lamp and coupler of the invention for use with spectroscopyinstruments intended to operate with a modulated light source.

As shown in the drawing, in practice the open end of the cylinder 14,the right hand end, is preferably provided with a safety window toprevent anyone from inadvertently sticking a finger into the highvoltage end of the coil 15 at start-up, and also to keep corrosivematerials out of the interior of the cylinder as much as possible. Thiswindow is suitably provided by a quartz window 34 mounted in a supportplug 35 fitted into the end of the cylinder 14.

What is claimed is:

1. A resonator coupler for coupling a source of r-f electric power intoan electrodeless discharge lamp for starting and operating the lamp,comprising:

a grounded hollow cylinder of electrically conductive material open atone end, with a grounded base member at the other end;

a helically coiled wire conductor concentrically within the cylinder andspaced from the inner walls thereof;

means for mounting a discharge lamp substantially concentrically withinone end position of the coil;

the wire of the coil being one quarter wave long relative to thefree-space wavelength of r-f power intended to be applied for operatinga lamp mounted therein;

the end of the coil at the end portion within which a lamp is adapted tobe mounted being toward said base member and being grounded, the otherend of the coil being open circuited; and

electrical connecting means for connecting to the coil a source of r-felectrical power that is sufficient to maintain a discharge in a lampmounted within the coil, said connecting means being tapped into thecoil at a point near, but spaced from, the grounded end thereof, saidpoint being selected such that, when a lamp mounted in the coil is inoperation by r-f power connected to the coil, the coupler is tuned tothe frequency of said r-f power and the impedance of said lamp andcoupling means at said tap point substantially matches the impedance ofsaid r-f power source, whereby when said r-f power is applied to thecoil, and before a discharge is ignited in the lamp, a voltage maximumoccurs at the open circuited end of the coil and creates a potentialextending through the lamp portion between said open circuited end andsaid base member for ionizing the gas in the lamp.

2. The coupler of claim 1 including a capacitor between the wall of saidcylinder and a turn of said coil for fine tuning the resonant frequencyof the resonator coupler relative to the frequency of an r-f source tobe applied thereto.

3. The resonator of claim 1 in which the outside diameter of the coil issubstantially one-half the inside diameter of the cylinder, the coilbeing of silver wire and the interior of the cylinder being polishedsilver plating.

4. The resonator of claim 1 which the open end of the cylinder extendsbeyond the open circuited end of the coil at least sufficiently to avoidfringe effects.

5. The resonator of claim 1 in which said base member extends across andsubstantially closes an end of the cylinder, the outside diameter of thecoil and inside diameter of the cylinder being substantially 1 inch and2 inches respectively, the wire of said coil being silver, and theinterior walls of the cylinder and base member being polished silverplating.

6. The combination of an electrodeless discharge lamp and a resonatorcoupler for coupling the lamp to a source of r-f electric power forstarting and operating the lamp, said coupler comprising:

a grounded hollow cylinder of electrically conductive material open atone end with a grounded base member at the other end;

a helically coiled wire conductor concentrically within the cylinder andspaced from the inner. walls thereof, said wire being one quarter wavelong relative to the free-space wavelength of r-f power intended to beapplied for operating the lamp, the end of the wire coil at the basemember end of the cylinder being grounded and the end toward the openend of the cylinder being open circuited, and the lamp being mountedwith at least the major portion of the lamp within the coil at thegrounded end of the coil; and

electrical connecting means for connecting to the coil a source of r-felectrical power that is sufficient to maintain a discharge in the lamp,said connecting means being tapped into the coil at a point near, butspaced from, the grounded end thereof, said point being selected suchthat, when the lamp is in operation by r-f power connected to the coil,

7. The combination of claim 6 in which said discharge lamp is atransparent sealed envelope containing an ionizing gas at low pressureand a minor amount of a metallic element for producing spectral light ofsaid metallic element.

8. The combination of an electrodeless discharge lamp for providingspectral light and resonator coupler for coupling the lamp to a sourceof r-f electric power for starting and operating the lamp;

said lamp being a transparent sealed envelope containing a minor amountof metallic element and an ionizing gas;

said coupler comprising:

a grounded hollow cylinder of electrically conductive material open atone end with a grounded base member substantially closing the other end;

a helically coiled wire conductor concentrically within the cylinder andspaced from the inner walls thereof, said wire being one quarter wavelong relative to the free-space wavelength of r-f power intended to beapplied for operating the lamp, the end of the wire coil at the basemember end of the cylinder being grounded and the end toward the openend of the cylinder being open circuited, the lamp being mounted with atleast the major portion of the lamp substantially concentrically withinthe coil at the grounded end of the coil, and the open end of thecylinder extending beyond the open circuited end of the coil to avoidfringe effects; and

electrical connecting means for connecting to the coil a source of r-felectrical power that is sufficient to maintain a discharge in the lamp,said connecting means being tapped into the coil at a point near, butspaced from, the grounded end thereof, said point being selected suchthat, when the lamp is in operating by r-f power connected to the coil,the coupler is tuned to the frequency of said r-f power and theimpedance of the lamp and coupler means at said tap point substantiallymatches the impedance of said r-f power source, whereby when said r-fpower is applied to the coil, and before a discharge is ignited in thelamp, a voltage maximum occurs at the open circuited end of the coil andcreates a potential extending through the lamp portion between said opencircuited end and said base member for ionizing the gas in the lamp.

9. The combination of claim 8 in which said lamp has a bulbous portionwhich is said major portion of the lamp, and an elongated stem portion,said base member having a bore therethrough, said stem being receivedthrough said bore, and releasable clamping means clamping the stem in aselected longitudinal position through the bore.

10. The combination of claim 8 in which the outside diameter of the coilis on the order of one-half the inside diameter of the cylinder, inwhich the interior surfaces of the cylinder and base member are polishedsilver and in which the wire of the coil is essentially silver.

11. A resonator coupler for coupling a source of r-f electric power intoan electrodeless discharge lamp for starting and operating the lamp,comprising:

a coiled wire conductor grounded at one end and open circuited at theother,

means for mounting a discharge lamp within the coil,

a generally tubularmember of electrically conductive materialsurrounding and spaced from the coil so as to be in a capacitorrelationship therewith,

said member being grounded and being open at one end,

the wire of the coil being one quarter wave long relative to thewavelength of r-f power intended to be cuited end of the coil andcreates a potential extending through the lamp for ionizing the gas inthe lamp.

12. The combination of an electrodeless discharge open circuited at theother, a discharge lamp mounted with at least a major portion within thecoil, a generally tubular shield of electrically conductive pp foroperating a p mounted in the Coil, material surrounding and spaced fromthe coil so as to be in a capacitor relationship therewith, saidelectrical connecting means for connecting to the Shield being groundedand being Open at one end coil a source of r-f electrical power that issufficient the wire of the coil being one quarter wave long rc|a tomalfltalrl a dlscharge m a lamp mounted W'thin tive to the wavelength ofr-f power intended to be the coil, said connecting means being tappedinto applied for Operating Said lamp and s f g ir g f i thelreofelectrical connecting means for connecting to the Sal emg 9 Sue w en aamp coil a source of r-f electrical power that is sufficient mounted inthe coil IS in operation by r-f power to maintain a discharge in thelamp, said connectconnected to the COll, the coupler lS- tuned -to themg means being tapped mto the coil at a pomt infrequency of said r-fpower and the impedance of termediate the ends thereot, said point beingsesaid lamp and coupling means at said tap pomt sub- 1 d h h h h l I bstantially matches the impedance of said r-f power ecte Sue t W S t eamp l mhoperdnlon source, whereby when said r-f power is applied to Mpower connecte to h col t e Coup l ls the coil, and before a dischargeis ignited in the tuned to the frequency ofsald r'fpower and h lamp, avoltage maximum Occurs at the open cip pedance of the lamp and couplermeans at said tap point substantially matches the impedance of said r-fpower source, whereby when said r-f power is applied to the coil, andbefore a discharge is ignited in the lamp, a voltage maximum occurs atthe open lamp and a resonator coupler for coupling the lamp to a sourceof r-f electric power for starting and operating tending through thelamp for ionizing the gas in the the lamp, said coupler comprising:lamp.

a coiled wire conductor grounded at one end and circuited end of thecoil and creates a potential ex-

1. A resonator coupler for coupling a source of r-f electric power intoan electrodeless discharge lamp for starting and operating the lamp,comprising: a grounded hollow cylinder of electrically conductivematerial open at one end, with a grounded base member at the other end;a helically coiled wire conductor concentrically within the cylinder andspaced from the inner walls thereof; means for mounting a discharge lampsubstantially concentrically within one end position of the coil; thewire of the coil being one quarter wave long relative to the free-spacewavelength of r-f power intended to be applied for operating a lampmounted therein; the end of the coil at the end portion within which alamp is adapted to be mounted being toward said base member and beinggrounded, the other end of the coil being open circuited; and electricalconnecting means for connecting to the coil a source of r-f electricalpower that is sufficient to maintain a discharge in a lamp mountedwithin the coil, said connecting means being tapped into the coil at apoint near, but spaced from, the grounded end thereof, said point beingselected such that, when a lamp mounted in the coil is in operation byr-f power connected to the coil, the coupler is tuned to the frequencyof said r-f power and the impedance of said lamp and coupling means atsaid tap point substantially matches the impedance of said r-f powersource, whereby when said r-f power is applied to the coil, and before adischarge is ignited in the lamp, a voltage maximum occurs at the opencircuited end of the coil and creates a potential extending through thelamp portion between said open circuited end and said base member forionizing the gas in the lamp.
 2. The coupler of claim 1 including acapacitor between the wall of said cylinder and a turn of said coil forfine tuning the resonant frequency of the resonator coupler relative tothe frequency of an r-f source to be applied thereto.
 3. The resonatorof claim 1 in which the outside diameter of the coil is substantiallyone-half the inside diameter of the cylinder, the coil being of silverwire and the interior of the cylinder being polished silver plating. 4.The resonator of claim 1 which the open end of the cylinder extendsbeyond the open circuited end of the coil at least sufficiently to avoidfringe effects.
 5. The resonator of claim 1 in which said base memberextends across and substantially closes an end of the cylinder, theoutside diameter of the coil and inside diameter of the cylinder beingsubstantially 1 inch and 2 inches respectively, the wire of said coilbeing silver, and the interior walls of the cylinder and base memberbeing polished silver plating.
 6. The combination of an electrodelessdischarge lamp and a resonator coupler for coupling the lamp to a sourceof r-f electric power for starting and operating the lamp, said couplercomprising: a grounded hollow cylinder of electrically conductivematerial open at one end with a grounded base member at the other end; ahelically coiled wire conductor concentrically within the cylinder andspaced from the inner walls thereof, said wire being one quarter wavelong relative to the free-space wavelength of r-f power intended to beapplied for operating the lamp, the end of the wire coil at the basemember end of the cylinder being grounded and the end toward the openend of the cylinder being open circuited, and the lamp being mountedwith at leaSt the major portion of the lamp within the coil at thegrounded end of the coil; and electrical connecting means for connectingto the coil a source of r-f electrical power that is sufficient tomaintain a discharge in the lamp, said connecting means being tappedinto the coil at a point near, but spaced from, the grounded endthereof, said point being selected such that, when the lamp is inoperation by r-f power connected to the coil, the coupler is tuned tothe frequency of said r-f power and the impedance of the lamp andcoupler means at said tap point substantially matches the impedance ofsaid r-f power source, whereby when said r-f power is applied to thecoil, and before a discharge is ignited in the lamp, a voltage maximumoccurs at the open circuited end of the coil and creates a potentialextending through the lamp portion between said open circuited end andsaid base member for ionizing the gas in the lamp.
 7. The combination ofclaim 6 in which said discharge lamp is a transparent sealed envelopecontaining an ionizing gas at low pressure and a minor amount of ametallic element for producing spectral light of said metallic element.8. The combination of an electrodeless discharge lamp for providingspectral light and resonator coupler for coupling the lamp to a sourceof r-f electric power for starting and operating the lamp; said lampbeing a transparent sealed envelope containing a minor amount ofmetallic element and an ionizing gas; said coupler comprising: agrounded hollow cylinder of electrically conductive material open at oneend with a grounded base member substantially closing the other end; ahelically coiled wire conductor concentrically within the cylinder andspaced from the inner walls thereof, said wire being one quarter wavelong relative to the free-space wavelength of r-f power intended to beapplied for operating the lamp, the end of the wire coil at the basemember end of the cylinder being grounded and the end toward the openend of the cylinder being open circuited, the lamp being mounted with atleast the major portion of the lamp substantially concentrically withinthe coil at the grounded end of the coil, and the open end of thecylinder extending beyond the open circuited end of the coil to avoidfringe effects; and electrical connecting means for connecting to thecoil a source of r-f electrical power that is sufficient to maintain adischarge in the lamp, said connecting means being tapped into the coilat a point near, but spaced from, the grounded end thereof, said pointbeing selected such that, when the lamp is in operating by r-f powerconnected to the coil, the coupler is tuned to the frequency of said r-fpower and the impedance of the lamp and coupler means at said tap pointsubstantially matches the impedance of said r-f power source, wherebywhen said r-f power is applied to the coil, and before a discharge isignited in the lamp, a voltage maximum occurs at the open circuited endof the coil and creates a potential extending through the lamp portionbetween said open circuited end and said base member for ionizing thegas in the lamp.
 9. The combination of claim 8 in which said lamp has abulbous portion which is said major portion of the lamp, and anelongated stem portion, said base member having a bore therethrough,said stem being received through said bore, and releasable clampingmeans clamping the stem in a selected longitudinal position through thebore.
 10. The combination of claim 8 in which the outside diameter ofthe coil is on the order of one-half the inside diameter of thecylinder, in which the interior surfaces of the cylinder and base memberare polished silver and in which the wire of the coil is essentiallysilver.
 11. A resonator coupler for coupling a source of r-f electricpower into an electrodeless discharge lamp for starting and operatingthe lamp, comprising: a coiled wire conductor grounded at one end andopen circuited at the other, means for mounting a discharge lamp withinthe coil, a generally tubular member of electrically conductive materialsurrounding and spaced from the coil so as to be in a capacitorrelationship therewith, said member being grounded and being open at oneend, the wire of the coil being one quarter wave long relative to thewavelength of r-f power intended to be applied for operating a lampmounted in the coil, and electrical connecting means for connecting tothe coil a source of r-f electrical power that is sufficient to maintaina discharge in a lamp mounted within the coil, said connecting meansbeing tapped into the coil at a point intermediate the ends thereof,said point being selected such that, when a lamp mounted in the coil isin operation by r-f power connected to the coil, the coupler is tuned tothe frequency of said r-f power and the impedance of said lamp andcoupling means at said tap point substantially matches the impedance ofsaid r-f power source, whereby when said r-f power is applied to thecoil, and before a discharge is ignited in the lamp, a voltage maximumoccurs at the open circuited end of the coil and creates a potentialextending through the lamp for ionizing the gas in the lamp.
 12. Thecombination of an electrodeless discharge lamp and a resonator couplerfor coupling the lamp to a source of r-f electric power for starting andoperating the lamp, said coupler comprising: a coiled wire conductorgrounded at one end and open circuited at the other, a discharge lampmounted with at least a major portion within the coil, a generallytubular shield of electrically conductive material surrounding andspaced from the coil so as to be in a capacitor relationship therewith,said shield being grounded and being open at one end, the wire of thecoil being one quarter wave long relative to the wavelength of r-f powerintended to be applied for operating said lamp, and electricalconnecting means for connecting to the coil a source of r-f electricalpower that is sufficient to maintain a discharge in the lamp, saidconnecting means being tapped into the coil at a point intermediate theends thereof, said point being selected such that, when the lamp is inoperation by r-f power connected to the coil, the coupler is tuned tothe frequency of said r-f power and the impedance of the lamp andcoupler means at said tap point substantially matches the impedance ofsaid r-f power source, whereby when said r-f power is applied to thecoil, and before a discharge is ignited in the lamp, a voltage maximumoccurs at the open circuited end of the coil and creates a potentialextending through the lamp for ionizing the gas in the lamp.